The present invention relates to a rechargeable lithium secondary battery which is usable as a transferable DC source and a buck-up electric source.
More particularly, it relates to a lithium secondary battery which is constructed of a non-aqueous electrolyte containing lithium ions, a positive electrode comprising a metal oxide, and a negative electrode comprising a metal oxide doped with lithium.
Lithium secondary batteries which can be charged and in which lithium is used as an active material of negative electrode have recently attracted much attention and have been extensively studied and developed because they have theoretically high energy density.
However, owing to dendritic or moss-like lithium produced during charging and discharging, positive and negative electrodes are in a state which can cause a so-called internal short circuit or result in the gradual collapsing of the shape of a negative electrode and the deterioration thereof. Therefore, it has been very difficult to attain a long charging and discharging cycle life.
One approach to solve this problem is to use a lithium alloy which can occlude and release lithium. But when a large amount of lithium is occluded and released, the alloy is apt to collapse and a sufficient effect cannot necessarily be obtained, therefore improvement of the negative electrode has still been desired.
As another approach to solve the problem, there is a battery system in which a vanadium pentoxide high in potential is used as a positive electrode and a niobium pentoxide doped with lithium is used as a negative electrode (see Japanese Patent Kokoku (Post. Exam. Publication) No. 62-59412). The niobium pentoxide used as a negative electrode in the battery can be easily doped and undoped with lithium and is considered to be considerably longer than the above-mentioned lithium alloy in charging and discharging cycle life.
As a result of various examinations on the above combination, the following problems have been found.
The first problem is as follows: When a battery is made using a mixture of vanadium pentoxide, carbon black as a conductive material and a fluorine-resin as a binder as a positive electrode, a mixture of niobium pentoxide doped with lithium, carbon black as a conductive material and fluorine-resin as a binder as a negative electrode and an organic solvent in which a lithium salt is dissolved as an electrolyte, a voltage of about 2 V can be obtained just after the construction. However, when the battery is stored in an atmosphere at a high temperature such as 60.degree. C., the voltage gradually decreases and simultaneously the selfdischarging proceeds, and finally the electric capacity is lost. Even if the battery is charged with a voltage of 2 V, the electric capacity is not sufficiently recovered and sometimes decreases to less than half of the initial capacity. Thus, even if the usual charging and discharging cycle life is long, no reliable cycle life can be expected for a long period and this must be improved.
Another problem is that a substance such as niobium pentoxide is low in processability and is not suitable for the mass-production of batteries and this poor processability must be improved.